A linear power amplifier requires the use of a high and a low side power transistor operating in a class B or AB mode. These power devices are usually driven by a low impedance voltage mode driver which must have sufficient drive capacity so that the large input capacitance of the power device does not affect the driver performance. This topology has several problems when using MOS transistors as the power devices, particularly in an integrated circuit application and/or when an all NMOS bridge is used. Among those problems are:
The gate driver must have sink and source capability since the input gate impedance is capacitive. In order to maintain a constant output current in the power device, the gate voltage must be held constant with zero current. If a class A gate driver is used, the result is very inefficient. If a class AB gate driver is used, higher efficiency is achieved, but crossover distortion in the driver results in non-linearity in the transfer function and possible loss of control under steady state conditions.
High gain and dominant pole compensation must be added to the input stage of the amplifier to make it usable in closed loop configurations. The integration of the compensation capacitor requires a relatively large amount of silicon area.
Power stage slew rate cannot be independently controlled. The slew rate of the power stage will either be controlled by the output impedance of the gate driver, or the internal compensation node. In either case, the slew rate cannot be independently controlled.
A floating voltage mode gate driver with low output impedance is needed to drive the high side power device. Since an NMOS transistor is much smaller (i.e., cheaper) than an equivalent PMOS transistor, it is very desirable to use NMOS transistors in both high and low side applications. Since the source of the NMOS transistor is floating when used as a high side power device, the gate driver must also be floating. This requires complex level shifting circuits in order to use a voltage mode driver.
It is an objective of the invention to provide a driver for a power MOS transistor that overcomes the foregoing disadvantages.